Building a Tree of Life

What is a tree of life?

A tree of life

The theory of evolution by natural
selection makes a powerful supposition: All species, past and present,
trace their ancestry to a single
common ancestor. If this is valid, and if evidence of common
ancestry can be found, scientists should be able to trace the evolution
of life on Earth. A tree of life is a powerful graphic model
that
is used to display this information.

To interpret a tree of
life is simple. Every node represents a common ancestor, from which
two different lineages branch.
Branches that occur closer to the top of the tree represent
lineages that
arose
later in evolutionary time. The tips of branches represent living
species. The closer two tips are within one branch, the more closely
they’re related.

How are trees of life built?

Scientists constructing
early trees of life used a variety of data to hypothesize relatedness
of modern species – mostly external
and internal features. This was supplemented with other types of
evidence, including the fossil record. It was (and is) particularly
exciting
for scientists to find transitional forms, which provide evidence
of common ancestry. One notable example is Archaeopteryx, which,
preserved as a fossil imprint, has both reptile and bird features,
and is used
to argue that birds evolved from reptiles.

Today, molecular evidence
in the form of DNA sequencing is used to provide data to build
a tree of life. The assumption with this
type of data is based on mutation — changes in DNA sequences
due to errors in replication. Mutation provides the variation upon
which natural selection acts in order to cause evolution. As two
species evolve from a common ancestor, their DNA will continue
to mutate and
will become less and less similar. Eventually, differences in DNA
become a genetic reflection of the differences between species.
DNA sequencing, therefore, is allowing scientists to build a
tree of life
that they consider to be much more accurate.